1. Field of the Invention
The present invention relates to a test method and a test device for a semiconductor circuit which constitutes, for example, an input/output buffer for a fast, small signal of HSTL (High Speed Transistor Logic) including a first semiconductor switch functioning as a pullup resistor and a second semiconductor switch functioning as a pulldown resistor, which are connected in series.
2. Description of Related Art
FIG. 10 is a circuit diagram showing a conventional input/output buffer of a fast, small signal. In FIG. 10, the reference numeral 101 designates a transistor functioning as a pullup transistor, and 102 designates a transistor functioning as a pulldown transistor. The two transistors are connected in series, with the other end of the transistor 101 connected to a device power supply 103, and the other end of the transistor 102 connected to a ground 104.
The reference numeral 105 designates a differential amplifier for comparing a voltage of a pad 111 connected to a connecting point 106 between the transistor 101 and transistor 102 with a reference voltage applied to a reference voltage input pad 107.
The fast, small signal input/output buffer with the foregoing configuration, having the embedded resistors, the pullup resistor and pulldown resistor of a particular fixed amount (50 ohms, for example), transfers a signal by current driving. Since the variations of the embedded resistors have an effect on the voltage amplitude of the transferred signal, it is necessary to test the quality of the fast, small signal input/output buffer or the semiconductor circuit by measuring the value of the embedded resistors.
One of the conventional method of such measurement will be described with reference to FIG. 11. First, the voltage and current across the transistor 101 are obtained by bringing the transistor 101 into conduction and the transistor 102 out of conduction by control signals fed from the control circuit 108, and by measuring a voltage across a voltmeter 110 connected between the connecting point 106 and ground 104 while causing a current to flow in the direction indicated by the solid line arrow by connecting a current source 109 between the connecting point 106 and ground 104. Second, the voltage and current across the transistor 102 are obtained by bringing the transistor 102 into conduction and the transistor 101 out of conduction by the control signals fed from the control circuit 108, and by measuring a voltage across the voltmeter 110 while causing a current from the current source 109 to flow through the transistor 102 in the direction indicated by the dotted line arrow. The values of the pullup resistor and pulldown resistor are computed from the measured currents and voltages.
The conventional test method and device for the semiconductor circuit has a problem of causing a measuring error in the resistances obtained from the measured voltages and currents, of an amount corresponding to a plus minus few tens of milliamperes, and it is difficult to limit the measuring error within a desired value of plus minus ten milliamperes even when using a high accuracy tester. This is because the measurement of the voltages and currents suffers from the effect of a contact resistance between the device pads and probe-card pointers in a wafer test, or a resistance 112 consisting of a contact resistance between the package and socket and wiring resistances in a final test.